Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

053
FXUS63 KLOT 212327
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
627 PM CDT Tue Apr 21 2026

.KEY MESSAGES...

- Threat for isolated to widely scattered thunderstorms this
  evening, mainly south/southeast of Chicago. Severe threat is
  low, but a few strong/gusty cores possible, particularly south
  of I-80 and east of I-55.

- Warming trend through Thursday (except cooler near Lake
  Michigan on Wednesday).

- More widespread showers and thunderstorms with another cold
  front Thursday night into Friday.

&&

.DISCUSSION...
Issued at 230 PM CDT Tue Apr 21 2026

Through Friday:

Early afternoon surface analysis depicts a weak surface low
pressure wave over central WI, along a cold-frontal trough which
trails southwest across IA and southern NE. This surface wave
is associated with a mid-level short wave trough evident in GOES
water vapor imagery tracking east-southeastward across the
region. Breezy southwest flow was in place across the warm
sector southeast of the front/wave, with gusts in the 30-35 mph
range (highest southeast cwa), helping to advect a narrow axis
of low-level moisture (surface dew points upper 40s/low 50s)
into the area. Despite this modest increase in low-level
moisture, soundings (including 18Z DVN RAOB) indicate low-level
capping beneath an inversion at the base of an EML plume.
Subtle moistening aloft at the base of the EML however, has
allowed for elevated convection to develop across parts of
southern WI and far north/northeast IL over the past few hours.
Various high-res CAMs have struggled with the general handling
of the elevated moisture axis and resulting footprint/coverage
of this convection, though empirically it appears that coverage
into the Chicago metro may be relatively isolated this
afternoon. While coverage may be limited, the steep lapse rates
within the EML plume and relatively dry sub-cloud base air mass
may support localized hail/gusty wind potential, especially from
the Chicago area south/southeast where CAMs continue to
indicate slightly higher coverage of storms. A few showers may
linger across our far south/southeast cwa into tonight.

The surface cold front to our northwest is progged to sag
slowly southeast across our northern cwa after midnight tonight,
eventually stalling in the vicinity of the I-80 corridor. High-
res RAP and HRRR runs have maintained a signal for fog/low
stratus to develop over Lake Michigan behind the front, which
may spread inland into adjacent northeast IL/northwest IN as
winds turn northeast off the lake. Not currently seeing much
evidence of this trend in satellite over the lake, this signal
has been consistent for several runs and thus have maintained
the mention of fog over the lake and patchy fog inland several
miles across the Chicago metro and I-80/94 corridors of
northwest IN.

On Wednesday, upper level ridging builds slowly east from the
Plains in the Mississippi River Valley ahead of a deep long-wave
trough and closed upper low developing across the western
CONUS. This induces southerly low-level flow across the Plains
and Mississippi Valley which eventually lifts the stalled front
back north as a warm front later Wednesday and Wednesday night.
Onshore flow near the lake will keep temperatures cooler (50s to
near 60) near the lake, while inland areas warm back into the
mid and upper 70s in the afternoon. With pooled moisture on the
warm side of the front, HRRR/RAP/ARW indicate some potential for
some isolated showers/thunderstorms across mainly our
southeastern cwa, where afternoon SBCAPE values increase to
500-750 J/kg. However, other than the presence of the boundary,
larger-scale forcing appears absent with rising heights
associated with the upper ridge and no notable short wave or
upper jet support. Thus have only slight (<20 percent) pops
during the late afternoon. The warm front continues to lift
north Wednesday night as the upper low over the western CONUS
lifts northeast across MT and broad southwest low-level flow
continues to edge eastward into the Mississippi Valley.

The warm front shifts north of the forecast area Thursday,
placing the forecast area within a breezy warm sector ahead of
the western trough. While some low-level moisture return is
noted, forecast soundings once again depict another EML plume
with moderately steep laps rates aloft, and an inversion based
around 800 mb providing capping through the day. Thus daytime
hours of Thursday appear to be precipitation-free at this point,
with breezy and warmer conditions as temperatures warm into the
lower 80s in most areas. Shower and thunderstorm chances then
look to increase Thursday night, as a mid-level short wave
rounds the base of the western upper trough, and
approaches/shifts across the forecast area. GFS/ECMWF MUCAPE is
greatest Thursday night, likely supporting a better thunderstorm
potential than during the day Friday, when a slow-moving cold
front pushes slowly east across the forecast area. Even so,
relatively limited instability, poor diurnal timing and weak
mid-level flow suggests the severe threat would remain fairly
low, especially considered to areas farther southwest of the
forecast area.

Ratzer

Friday night through Tuesday:

As an upper level trough exits to the east-northeast, rain
showers may linger through the overnight, but eventually
dissipate through the early morning hours. The surface cold
front (mentioned in the discussion above) will continue to move
through the area Friday night through Saturday morning, dropping
morning low temperatures into the 40s. A cooler air mass and
persistent north-northeast winds will help limit high
temperatures through the weekend to the 60s north of I-88, and
the 50s along the lakeshore. Weak upper level
ridging over the weekend should make for drier conditions on
Saturday and Sunday.

As shortwave trough over southern California on Sunday morning
is expected to move northeastward and phase with the larger
upper level low parked over Alberta early next week. There is
pretty decent model agreement in surface cyclogenesis developing
in the Colorado front range Sunday night before moving
northeast across the Plains toward the Great Lakes early next
week. While confidence in timing is still limited at this
distance, there is a growing signal for the likelihood for
showers and storms to pass over the forecast area early next
week. The exact track and depth of the low will also have to be
monitored in the coming model runs. That can help influence the
duration and how strong the showers and storms may become.
Seasonable temperatures are forecasted through the middle of
next week behind the system.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 627 PM CDT Tue Apr 21 2026

Forecast concerns include...

Ifr/Lifr cigs/vis with fog overnight/Wednesday morning.
Wind shift to northeast overnight.

Scattered showers and thunderstorms currently along and south of
an IKK/SBN line will continue moving east/southeast and away
from the terminals this evening.

Southwest winds may gust into the 20kt range for the next 1-2
hours and will quickly diminish after sunset, possibly becoming
light and variable later this evening. Winds will shift to the
northeast with a weak cold front overnight and then remain
northeast or easterly Wednesday for the Chicago terminals, more
southeasterly elsewhere.

The cold front will allow ifr/lifr cigs and possibly dense fog
to form over Lake Michigan. Some of this is expected to move
inland across the Chicago terminals but only medium confidence
and opted to make no changes to the previous forecast. Trends
will need to be monitored for possible lower vis at ORD/MDW.
Vis/cigs should steadily improve through mid/late morning over
land but the fog is expected to persist over Lake Michigan and
possibly move back inland Wednesday evening. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 7 PM CDT this evening for the IN
     nearshore waters.

&&

$$

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